win32.h: declare fstati64 to suppress warnings

* include/ruby/win32.h (fstati64): declare for ext/-test-/file/stat.c
  on w64-mingw, to suppress warnings.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43281 b2dd03c8-39d4-4d8f-98ff-823fe69b080e